So, what would a protein-rich, low-carbon diet really look like? Just how bad for the climate are meat and dairy? How much more sustainable is it to only eat plant-based proteins, such as tofu, chickpeas and peas? Is it better to cut out cheese or chicken? Which animal-free alternatives have the lowest emissions’ output?

BBC Future set out to answer these questions, using data from the largest ever analysis of food systems, compiled by Joseph Poore, a researcher at the University of Oxford, and Thomas Nemecek, who studies the lifecycle of food at Swiss research institute Agroscope.

Meat from small, non-ruminant animals, such as chicken, turkey, rabbit and duck, has a much lower GHG footprint than beef and lamb. Chicken, for example, has a GHG footprint almost nine times lower than beef’s – generating 5.7kg of CO2e per 100g of protein.

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It is cheese, not chicken or pork, that generates the third-highest emissions in agriculture, after lamb and beef.

“There’s this consensus that ‘being vegetarian is great’, but then we sort of forget that cheese is actually pretty carbon intensive,” says Marbach, noting that this is due to cows’ high methane output and the fact that they require “a lot of inputs for not much output”.

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